Yeah it's weird pixies can caste polymorph, like why do they need that spell at their CR. I don't think it makes sense for lore ether.
A way to fix the banishment type effect is make it so it can only be cast on a creature who is not a threat (like being tied up, unaware, or several levels below you) so its more balanced and keeps the idea of a witch/wizard curseing someone to become a frog. Scouting I'm fine with because it is a common fantasy trope. My only solution for being a combat buff is capping the health you can have per level or stronger forms have a lesser duration.